The present invention relates to the field of communications, and, more particularly, to radar systems and related methods.
Radio detecting and ranging, or radar as it is commonly known, is used for a wide variety of applications. For example, radars are used for tracking the position of airplanes, monitoring atmospheric conditions, and mapping terrain. Basic radar systems operate by transmitting a waveform, usually in the form of a pulse, at a target. The time between transmission and reception of signals reflected from the target provides a range measurement, i.e., a measurement of the line of sight to the target. Furthermore, the angle-of-arrival of the reflected signals also provides an azimuth measurement of the target which is orthogonal to the range direction.
To achieve fine azimuth resolution, a large antenna may be required to properly focus the transmitted and received signals. Moreover, if relatively low frequencies, such as UHF or VHF frequencies, are used by the radar, the size of the antenna required to obtain desired azimuth resolution will be even larger. Thus, when the radar is to be carried by an airborne or spaceborne platform (e.g., an airplane or satellite), the antenna size required to achieve sufficient azimuth resolution may be prohibitive.
For such applications, a different radar technique has been developed called synthetic aperture radar (SAR). Rather than use an unmanageably long antenna to obtain the desired azimuth resolution, SAR takes advantage of relative movement between the radar system and the target. In the case of an airborne or spaceborne platform, the radar system moves over targets below the platform, which may or may not be stationary.
Basically, a SAR collects reflected signals over an extended distance or length of travel of the platform and processes this data as if it came from a much longer antenna. That is, the distance the platform travels is referred to as the synthetic aperture, which yields greater resolution than would otherwise be possible simply based upon the actual physical size of the antenna. Of course, the same approach may essentially be used in reverse for tracking moving targets using a stationary radar system, which is known as an inverse SAR (ISAR).
Despite the advantages of SAR, its unique characteristics also present several challenges in processing the reflected signals. For example, due to the Doppler effect caused by the relative movement between the radar system and target, range/Doppler ambiguities result. Larger scene extents in azimuth require higher pulse repetition frequencies (PRFs) to measure the larger Doppler spread of the reflectivity measurements. However, larger range extents of the scene require lower PRFs to avoid range ambiguities due to simultaneous returns from more than one transmit pulse.
In order to achieve a small enough scene size in range and azimuth that allows a workable PRF, traditional SAR design calls for a narrow antenna beam and, therefore, a large antenna area. The antenna area requirement for unambiguous SAR increases versus wavelength such that lower frequency SAR is more difficult. Also, the required antenna area increases dramatically for shallower grazing angles. This problem is also exacerbated for spaceborne SAR due to the high orbital velocity and long stand-off range.
Various prior art SAR techniques have been developed to address some of these problems. By way of example, U.S. Pat. No. 5,815,111 is directed to a method of defocusing range ambiguities in a pulse radar, such as a SAR. The method includes spreading radar pulses at transmission using a plurality of xe2x80x9cchirpxe2x80x9d rules for varying the frequency of the transmitted wave as a function of time. During the transmission of successive pulses, chirp rules are alternated between chirps that rise and chirps that fall in the frequency/time plane of the pulse. Furthermore, received echoes are compressed by matched filtering using a correlation operation between the received echo signal and the chirp rule that was applied at the time of transmission of the pulse which gave rise to the echo signal. U.S. Pat. No. 5,926,125 describes a similar technique for suppressing ambiguities using alternating swept frequency pulses.
U.S. Pat. No. 5,808,580 is directed to a waveform/signal processing concept that uses a train of frequency coded pulses. The pulses include the same frequencies but have a different time-frequency ordering. This patent also notes that phase-coded pulses may optionally be used. A stack of correlators are also disclosed for providing the subsequent pulse compression.
The above prior art all allow an N-times higher PRF, and therefore an N-times smaller antenna area, while suppressing range-Doppler ambiguities. However, significant cross-correlation artifacts remain after the correlation/matched filtering operations. The correlation operation against a subpulse 1, for example, will not only produce an auto-correlation result for the subpulse 1 signal but will also produce cross-correlation artifacts for the other Nxe2x88x921 subpulse signals that are simultaneously returning. These artifacts (i.e., sidelobes) produce what is known as multiplicative noise since their amplitude scales linearly with target signal amplitude. This multiplicative noise severely degrades the image quality and observable scene dynamic range. For larger values of N the scene measurement can be rendered unusable with these prior art approaches.
Furthermore, the above-noted prior art approaches all use a train of N pulses that implicitly are repeated until the required processing interval has been measured. Yet, the periodic repetition of each of the N waveforms produces cross-range (i.e., azimuth) sidelobes that also degrade the image quality.
U.S. Pat. No. 5,745,069 also describes a prior art approach using a train of N subpulses. This invention considers frequency-orthogonal pulses and essentially trades frequency bandwidth for ambiguity suppression. The radar bandwidth is N-times higher, and the required antenna area is N-time less, but the range resolution corresponds to the fractional 1/N subpulse bandwidth.
Certain prior art approaches have addressed SAR image sidelobe artifacts by using a CLEAN algorithm, which is a form of iterative deconvolution. For example, U.S. Pat. No. 5,394,151, which is directed to an apparatus and method for producing three-dimensional images, estimates the location and complex strengths of scatterers and uses these estimates to generate a side lobe free image using a CLEAN algorithm. Another similar method and apparatus for processing radar images using a CLEAN algorithm is disclosed in U.S. Pat. No. 5,383,457. These approaches may effectively remove auto-correlation (impulse response) sidelobes, but they use single frequency waveforms and thus do not address cross-correlations caused by multiple frequency waveforms.
In view of the foregoing background, it is therefore an object of the present invention to provide a radar system, such as a synthetic aperture radar (SAR) system, for example, which provides desired range and azimuth resolution and allows high PRF operation with smaller antennas while removing auto and/or cross-correlation artifacts.
This and other objects, features, and advantages in accordance with the present invention are provided by a radar system which may include an antenna, a waveform generator for generating a plurality of waveforms having different frequency components, and a transmitter connected to the waveform generator for transmitting the plurality of waveforms via the antenna. Moreover, the radar system may also include a receiver connected to the antenna for receiving reflected signals from targets, and a processor for iteratively deconvolving the reflected signals to generate target data.
The use of waveforms having different frequency components advantageously allows an increased pulse repetition frequency (PRF) to be used to resolve an extended Doppler footprint, as resulting range ambiguities and/or auto/cross-correlation artifacts may be removed through the iterative deconvolution processing. By way of example, the waveform generator may pseudorandomly select the different frequency components for the waveforms, as well as generate the plurality of waveforms such that at least some of the waveforms are orthogonal to one another. This causes the resulting ambiguities to be spread in range and cross-range. The different frequency components may also be contiguous to define stepped frequency pulses or spaced apart in frequency.
By way of example, the processor may iteratively deconvolve the reflected signals using a CLEAN algorithm which selects a brightest target from among the reflected signals and moves the target peaks to a xe2x80x9ccleanxe2x80x9d image with no noise background. The side lobes or artifacts from the target are then deconvolved to reveal a next brightest target, and the foregoing process continues until all of the desired targets have been distinguished from the reflected signals. In particular, such artifacts may be caused by both auto-correlation and cross-correlation. In accordance with the invention, the processor may iteratively deconvolve the reflected signals to remove auto and/or cross-correlation artifacts.
Additionally, the antenna may advantageously operate using different polarizations (e.g., right and left-hand circular polarizations, horizontal and vertical polarizations, etc.), and the waveform generator may generate waveforms for the different polarizations. Moreover, an encoder may be connected between the waveform generator and the transmitter for encoding the waveforms using identification codes (e.g., pseudorandom number codes) corresponding to the different polarizations. Further, the transmitter may simultaneously transmit the encoded waveforms for the different polarizations, and the processor may separate different reflected polarization signals based upon the identification codes prior to iteratively deconvolving the reflected signals. Thus, since separate transmission cycles are not required for transmitting the different polarizations, no increase in PRF is required and temporal decorrelations may be avoided.
More particularly, the radar system and targets may be relatively movable with respect to one another, and the processor may therefore store reflected signals over a length of relative movement and generate the target data based upon the stored signals to thus provide a synthetic aperture radar (SAR) system or an inverse SAR (ISAR). The antenna may be a phased array antenna, for example.
A method aspect of the invention is for generating target data and may include generating a plurality of waveforms having different frequency components and transmitting the plurality of waveforms via an antenna. The method may further include receiving reflected signals from targets via the antenna and iteratively deconvolving the reflected signals to generate the target data.
More particularly, generating may include pseudorandomly selecting the different frequency components for the waveforms. Generating the waveforms may also include generating different frequency components that are contiguous to define stepped frequency pulses, or generating different frequency components that are spaced apart in frequency. The plurality of waveforms may also be generated such that at least some of the waveforms are orthogonal to one another.
Additionally, the antenna may operate using different polarizations, and generating may further include generating waveforms for the different polarizations. By way of example, the different polarizations may be horizontal and vertical polarizations, as well as right-hand and left-hand circular polarizations. The waveforms for the different polarizations may also be simultaneously transmitted.
The method may also include encoding the waveforms using identification codes corresponding to the different polarizations, and separating different reflected polarization signals based upon the identification codes prior -to iteratively deconvolving the reflected signals. For example, the identification codes may be pseudorandom number codes.
Furthermore, the iterative deconvolution may include iteratively deconvolving the reflected signals to remove auto and/or cross-correlation artifacts. Also, the radar system and targets may be relatively movable with respect to one another, reflected signals may be stored over a length of relative movement, and the stored signals may be processed to generate the target data.